Semiconductors form the basis of modern electronics. Possessing physical properties that can be selectively modified and controlled between conduction and insulation, semiconductors are essential in most modern electrical devices (e.g., computers, cellular phones, photovoltaic cells, etc.).
The ability to deposit semiconductor materials using non-traditional semiconductor technologies such as printing may offer a way to simplify and hence reduce the cost of many modern electrical devices (e.g., computers, cellular phones, photovoltaic cells, etc.). Like pigment in paint, these semiconductor materials are generally formed as microscopic particles, temporarily suspended in a matrix, colloidal dispersion, or paste, and later deposited on a substrate.
For example, it may be desired to form an electrically active thin film (i.e., p-n junction, etc.) on a silicon substrate through the deposition of fluid with electrically active semiconductor particles. In order to have the fluid flow during the deposition process (e.g., inkjet print, aerosol print, screen print, etc.), it generally must have a relatively low viscosity. However, once deposited in a pattern, the same fluid must generally also have a relatively high viscosity in order to retain the pattern during the densification process. Consequently, a shear-thinning or non-Newtonian fluid is desired. That is, the fluid is rigid for shear stress τ, less than a critical value τ0. Once the critical shear stress (or “yield stress”) is exceeded, the material flows in such a way that the shear rate,
      ∂    u        ∂    y  is directly proportional to the amount by which the applied shear stress exceeds the yield stress:
                                          ∂            u                                ∂            y                          =                  {                                                                      0                  ,                                                                              τ                  <                                      τ                    0                                                                                                                                                                  (                                              τ                        -                                                  τ                          0                                                                    )                                        /                    μ                                    ,                                                                              τ                  ≥                                      τ                    0                                                                                                          [                  EQUATION          ⁢                                          ⁢          1                ]            
Typically, non-Newtonian behavior may be achieved by combining in a suitable solvent, a set of high molecular weight (HMW) molecules, and a certain critical volume fraction of granular material (particles). Below the critical volume fraction, the fluid is Newtonian, whereas above the critical volume fraction, the fluid is non-Newtonian or shear-thinning.
However, in order to optimize patterned deposition, it is often desired to deposit a fluid with a substantially low volume (typically less than 10 wt %) of granular materials, which may be below the critical volume fraction for shear-thinning behavior. For example, in the deposition of thin films, higher loadings of sub-micron particles tend to form thicker films (>5 micron) which, in turn, tend to develop stress fractures when the film is densified. Furthermore, in applications where an epitaxy is required, thicker films tend to impede epitaxial growth. An epitaxy is generally a type of interface between a thin film and a substrate and generally describes an ordered crystalline growth on a mono crystalline substrate.
In view of the foregoing, there is desired a shear thinning Group IV based nanoparticle fluid with a sub-critical volume fraction.